A voltage controlled oscillator (VCO) is an oscillator with output frequency that varies ideally as a linear function of an input dc (or slowly varying voltage). In practice the variation is not perfectly linear and output frequency can be different from the ideal linear value desired. The difference may be called frequency errors and the errors arise from nonlinearities in the VCO's "characteristic" frequency/voltage response function.
One of the most useful applications of VCO's is in generating linear frequency sweeps. Typically the input to the VCO is a periodic sequence of voltage ramps resembling a sawtooth function. For each linear portion of the input signal the output VCO frequency should vary from some minimum value to some maximum value in ideally a linear manner. The same instantaneous frequency behavior would repeat for each frequency sweep interval. In practice the "linear" sweep contains errors because of the nonlinearities in the VCO's characteristic. Since the sweep voltage is periodic and the VCO characteristic does not change from sweep-to-sweep, the instantaneous frequency errors are periodic. This fact is important to the ideas embodied in the new invention.
Several ways exist for reducing the sweep nonlinearities. Of these, those that utilize a closed loop approach provide the most improvement. Of the closed-loop methods available, the phase-locked loop will generate about the most linear frequency sweeps possible.